1. Field of the Invention
The present invention generally relates to cables having the double function of earth conductors and optical telecommunications cables. More particularly, the invention relates to earth cables including optical fibers for telecommunications purposes, and which are laid underground.
2. Description of the Related Art
Some electrical apparatus, for example wind mill towers for power generation, can make use of different cables for interconnecting such apparatuses to an electrical network and for services associated thereto. In particular, electrical cables, typically medium voltage cables, allow the power transport—generated power, in the case of a wind mill tower—; earth cables are connected to the earth system of the apparatus or of the network to which such apparatuses belong, and to the ground; optical cables are also used to provide communication or telecommunication services, as well as for the control of the apparatus operation and other functions. Typically, the three cable types above are at least partially laid underground, conveniently in the same trench.
In principle, earth cables can have a very simple structure, generally comprising, but also merely consisting of, a metallic conductor, typically in form of stranded wires. The metallic conductor is preferably a copper conductor. An example of such prior art earth cable according is depicted in FIG. 4.
Optical cables have a structure determined by their location and by the need to provide a suitable protection to the optical fibers contained therein. The optical fiber sensitivity to mechanical stress together with the environmental conditions, such as presence of water and/or chemical agents that could penetrate into the cable, and attack by animals such as rodents are examples of the requirements which have to be taken into account in the design of such cables.
The cable laying method comprises the positioning of the cable in a sand bedding; the sand bedding is then covered by a layer of ground based on small particle dimension material (for example, selected from the trench excavation material), covered, in turn, by common ground (typically, the remainder of the trench excavation material). This process is cumbersome and time-consuming in case many different cables are to be laid into the trench.
Cables comprising an earth conductor and optical fibers are known for aerial use. Such cables, usually called optical ground cable wires (OPGWs), typically comprise aluminum and/or stainless steel tubes housing the optical fibers combined with wires of aluminum or reinforced aluminum, for example aluminum clad steel and/or aluminum alloy, to provide short-circuit capacity and further provide the cable with mechanical strength as required for overhead installation. An example of OPGW is described in EP 1210633. Such cables are not suitable for use underground, for many reasons, including the fact that the presence of aluminum tubes housing the fibers would result in corrosion and in possible formation of hydrogen, which would accumulate in time inside the tubes, causing consequent worsening of the performance of the optical fibers.
The use of aerial ground cable wires is not possible or convenient in most cases, for example when wind mills are present, because possible aerial lines would disturb the location of the wind mills towers. In addition, absent an overhead power line, to which they could be associated with, they would need independent poles, which would increase the cost and complexity of the system.
The Applicant has observed that a simpler installation could be provided by combining an underground earth cable with a communication cable comprising optical fibers. In particular, it has been found that, by inserting an optical element containing optical fibers into an assembly of electrical wires suitable for providing ground connection, the electrical wires can act as armour for the optical element, thereby providing suitable protection from the soil where the cable has been laid.
Such earth conductor suitable to be laid underground should have a structure satisfying diverse requirements. A sufficient mechanical protection to the optical fibers contained in the cable is a must. At the same time, the optical fibers should be easily extractable from the cable at any point of its length—not only at the heads thereof—for effecting foreseen and unforeseen connections to premises. Furthermore, a suitable protection for the optical fibers should be provided from temperature increases due to the current passing trough the electrical wires
A robust mechanical protection could allow an easy laying of the cable underground, too.
In view of the environment, the cable should also be resistant to attacks by water and/or chemical agents and/or by animals like rodents.
As for the electrical connection of the earth cable to the ground system, it can be realized with clamps, but the use of welding is usually preferred because more economical on a long term basis. During the welding a substantial heat is generated, possibly harmful for the optical fibers that are generally tested for temperature not higher than 60° C. A thermal protection for the optical fibers contained in the cable is desirable.